Miami-Dade’s toll authority will once again offer rebates for frequent drivers, extending a program enacted amid a backlash against toll hikes along the county’s busiest highways.

The Miami-Dade Expressway Authority — best known as the MDX — announced Tuesday its board approved a 30 percent rebate for customers already enrolled in the rebate program. Spokesman Mario Diaz said the checks will go out in December, with an average amount of $100.

Eligible MDX customers don’t have to do anything to receive the money, which comes from a $5.5 million pool of surplus cash from the 2016 budget year, which ended June 30. The MDX relies on tolls to maintain five highways, including State Road 836 expressway, called the Dolphin Expressway, and State Road 112, called the Airport Expressway.

There were enrollment periods in 2015 and 2016 for the toll-rebate program. About 112,000 customers signed up for the program, which kicks in once a customer pays at least $100 in tolls to the government agency during the 2016 budget year.

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Diaz said any customer who received a check when the program started in 2015 was automatically in the running for the latest batch of rebates, but would still need to meet the $100 threshold It’s too late to register for the checks now, though.

Critics, including Miami-Dade Mayor Carlos Gimenez, pointed to the registration process as a reason why MDX would be better off just lowering tolls for everybody rather than writing yearly checks to frequent users. Diaz said putting the rebates toward lowering tolls instead would only knock about a penny off the rates.

MDX officials said the expansion made the tolling system more fair, since the prior set-up allowed some motorists to ride the expressways for free while others paid the tolls that cover maintenance and debt.

Diaz said there’s no cap on the rebate amount, and that one air-conditioning company earned a $1,700 rebate last year for its fleet. About 54,000 motorists should be receiving checks in 2016, roughly half of the motorists who enrolled in the program.

The $5.5 million payout amounts to a tiny expense for the agency, which is run by a board of state and county appointees. In 2016, MDX expected to collect $202 million in tolls, according to budget documents.